Here's the near final version of section 3
(3) Comparison of proxy records to GCM model outputs
3.1 Description of GCM model experiments
We use output from two state-of-the-art experiments using two (UK and German) coupled
ocean-atmosphere GCMs in which best estimates of (i) natural forcings only
(ii) natural plus anthropogenic radiative forcings are applied over the
past 500 (UK) to 1000 (German) years. By using results from two GCMs we will be able to
quantify some of the uncertainty in past climate simulations associated with
model climate sensitivity. The GCMs are HadCM3 (Gordon et
al, 2000, Pope et al, 2000) and ECHO-G (Zorita et al., 2003). Output from
HadCM3, will be made available by the Hadley Centre. The first experiment
(NAT500) simulating the period 1500 to 2000 is driven with natural
forcings alone (changes in solar irradiance, volcanic aerosol and orbital
parameters (mainly the date of perihelion). In this experiment land cover
and greenhouse gases are set to pre-industrial conditons. The second
experiment (ALL250) over the period 1750-2000 includes all natural
forcings plus additional anthropogenic forcings. These are the changes in well-mixed
greenhouse gases , sulphate aerosols
(both their direct effect on scattering solar radiation and their indirect
effect on cloud albedo), changes in vegetation (mainly the replacement of
forest with pasture) and changes in tropospheric and stratospheric ozone.
Simon Tett at the Hadley Centre will provide data
on near surface temperature and other diagnostics. The ECHO-G experiments
are very similar except that there is no land use change , geographical structure to
volcanic aerosol forcing and no specification of tropospheric aerosols.
The 'natural forcings only' experiennt runs for 1000 years to the present.
A very preliminary analysis suggests that anthropogenic forcings have had
a substantial effect on Arctic Siberian climate during the 20th century.
For the illustrative study region the ALL250 simulation indicates a
warming of about 0.750C since 1950 compared to the NAT500 simulation,

Prior to proxy-model comparisons we will determine the extent to which the
HadCM3 and ECHO-G GCMs accurately reproduce (a) the basic state of the
regional climate (b) the timescales and structure of regional climate
variability and the associated dynamics. We will undertake an equivalent
analysis to that with observational data described in Section 1.4 using
data from the GCM control and experimental runs and compare with results
from the analysis of observations.
3.2.2. Paleo data-model comparison
We will test the veracity of the simulated model climate variability
against proxy data by quantifying the coherence in regional climate
variability estimates from HadCM3 and ECHO-G and multi-proxies at
decadal-centennial timescales. We will compare the regional climate
history in the GCMs and proxies in terms of (a) the magnitude of variance
in (b) the relative variance at different timescales (c) the phase
relationships and coherence of variability. This will employ a variety of
timeseries approaches (including cross spectral analysis and wavelet
analysis),correlation and regression. We will utilise techniques for
proxy-model comparison developed within the EU-funded SO&P projects
(Simulations, Observations & Paloclimatic data: climate variability over
the last 500 years) which account for the error in the GCM and proxy
estimate. This will determine the statistical uniqueness of recent trends
relative to past periods. We will also determine whether the spatial
pattern of lake response bwteen the sites is consistent with that
recorded in the spatially more complete tree ring data and the GCM output.
Using the timing of climate response to the natural and anthropogeic
forcing history as simulated by the GCMs we will be able to attribute
causes to variability observed in the proxy records. From our analysis we
will determine whether the observed lake ecosystem response is more
consistent with climate variability/change simulated by GCMs under natural
or natural plus anthropogenic forcing.
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Martin Todd
University Lecturer
Department of Geography
UCL (University College London)
26 Bedford Way
London WC1 8HR
email m.toddatXYZxyzg.ucl.ac.uk ********************************